phi

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A lean, high-performance terminal coding agent harness in Go — a sibling to Pi.

Docs: pulseaiclub.github.io

• Fast and small — ~15 MB release binary, ~21 MB idle RSS, ~31 ms to first frame; no Node / Electron / Python runtime
• Sub-agents — spawn isolated jobs and watch the full run unfold in the TUI / job logs, without stuffing every turn into the parent context
• Hashline edits — edit by whole-file @file path#TAG plus line LINE#HASH anchors (same idea as oh-my-pi): the model points at anchors instead of rewriting whole files; stale tags/hashes are rejected so over-edits and silent corruption stop here
• Permission gate — Gate / Ask before destructive tools fire; safety is not optional when an agent can touch your tree
• MCP without context death — configure as many MCP servers as you want; their tool schemas never enter the model prompt. The system prompt lists server names only (like the Skills catalog); the agent uses three meta-tools (mcp_list / mcp_inspect / mcp_call) to discover and call on demand. Same Gate / Ask / Hooks path as built-in tools. See MCP
• Extensions (Go or Rust) — native binaries speak the PXB binary protocol over stdin/stdout; official author SDKs for Go (ext/go) and Rust (ext/rust): LLM tools, slash commands, event intercepts, confirm dialogs — no reflection; JSON at the SDK edges via serde_json. See Extensions
• In-TUI diff review — /diff opens a full-screen git review (working tree / staged / HEAD): syntax-highlighted hunks, line notes, then a sends notes to the agent. See Diff review
• Any model — OpenAI-compatible or Anthropic, no vendor lock-in

• Docs
• Quick start
• Footprint
• Configuration
• Interactive mode
• Diff review
• Commands
• Sessions
• Headless mode
• Skills
• Permissions
• Extensions
• MCP
• Tools
• Project layout

Quick start

Install the latest release (macOS / Linux):

curl -fsSL https://raw.githubusercontent.com/pulseaiclub/phi/main/scripts/install.sh | bash

Windows (PowerShell 5.1+):

irm https://raw.githubusercontent.com/pulseaiclub/phi/main/scripts/install.ps1 | iex

First launch needs a model. Open the config editor (creates ~/.phi layout and writes ~/.phi/config.yaml):

phi config

Or set env vars for a one-off run:

export PHI_MODEL=gpt-4o
export PHI_API_KEY=sk-...

Then start the TUI:

phi

Or build from source (Go 1.26.3+, see go.mod):

make build          # produces ./phi
make install        # build and install into $GOBIN

On first start, phi automatically creates ~/.phi/{bin,skills,hooks,session}. Search tools (fd, rg) download into ~/.phi/bin in the background when missing.

The TUI gives the model four core tools — read, write, edit, and bash — plus grep, find, and ls. The model uses these to fulfill your requests. External HTTP fetch is available via MCP when configured.

Footprint

Lean is not enough — phi is built to feel instant and stay cheap under load. phi numbers are a stripped release build (CGO_ENABLED=0, -ldflags="-s -w") on macOS arm64. Other harnesses use published Linux PSS / interactive PTY figures.

Time to first frame

Idle RAM · 1 session

Idle RAM · 10 sessions

Configuration

phi reads ~/.phi/config.yaml (standard YAML). Environment variables override it for one-off runs. phi config opens an HTML editor for the same file in your browser.

# ~/.phi/config.yaml
models:
  - name: gpt-4o            # model name; "claude-*" routes to the Anthropic API
    api_key: sk-...         # or set PHI_API_KEY
    base_url: https://api.openai.com/v1   # default; PHI_BASE_URL overrides
    context_window: 128000  # optional
    default: true           # the model used at startup; first entry wins if absent
  - name: claude-sonnet-4-20250514   # extra models; switchable at runtime
    api_key: sk-ant-...
    base_url: https://api.anthropic.com
    context_window: 200000

skill_path: ~/.phi/skills # where SKILL.md files are loaded from

agents:
  enabled: true           # default; set false to disable agent_* sub-agent tools
  models:                 # optional per-role defaults; omit → inherit parent model
    explore: cheap-model
    review: strong-model
    worker: coding-model

permissions:
  mode: interactive       # interactive | readonly | autopilot | headless-strict
  bash:
    default: ask          # ask | allow | deny
    allow:
      - "go test ./..."
    deny:
      - "rm -rf *"

Recommended model: DeepSeek Flash

phi + DeepSeek Flash — the best pairing: grounded, low hallucination, cache hit rates near 100%.

Measured data:

39 LLM rounds, same session — prompt 16k→40k, hit rate 95–100% (avg 98.7%).

xychart-beta
    title "Cache hit % (39 rounds)"
    x-axis [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39]
    y-axis "Hit %" 95 --> 100
    line [98.1, 98.8, 97.5, 98.1, 98.1, 98.8, 99.1, 99.9, 98.7, 99.7, 99.5, 99.5, 96.3, 99.4, 97.0, 99.7, 99.9, 99.3, 98.3, 96.4, 98.9, 98.7, 97.2, 99.8, 95.0, 99.7, 99.2, 98.9, 99.3, 99.5, 99.5, 99.4, 99.2, 98.6, 99.4, 99.6, 100.0, 97.8, 99.4]

Environment overrides:

Provider routing: a base URL containing anthropic or a model name starting with claude uses the Anthropic Messages API; everything else uses the OpenAI-compatible /chat/completions path.

Workspace layout

~/.phi/
├── config.yaml   # global configuration
├── bin/          # downloaded search tools (fd, ripgrep)
├── skills/       # SKILL.md skill directories
├── extensions/   # PXB binaries + phi.yaml
├── jobs/         # sub-agent job artifacts (meta, logs, result.md)
└── session/      # persisted sessions, one dir per working directory
    └── <encoded-cwd>/

Interactive mode

phi (or phi tui) starts the TUI: a chat transcript on top, an editor at the bottom, and a footer with the current activity. When a newer release is available, the footer shows a hint like 0.2.0 available · phi update.

Assistant output is rendered as Markdown (CommonMark/GFM): headings, emphasis, strikethrough, links, blockquotes, lists, task checkboxes, and tables are styled with the active theme; fenced code blocks get a muted language caption and per-language syntax highlighting. Structural markers (#, `, *) are stripped.

The editor supports:

• @ — fuzzy file mention picker (type @ and start typing a path)
• / — slash command picker (/sessions, /resume, /clear, /diff)
• ? — shortcut help picker (lists /, !, @, and key bindings; Esc closes)
• !command — run a shell command locally and stream its output into the transcript (see Commands)
• Ctrl+K — command palette: settings → model / theme